1. Field of the Invention
This invention relates to a process and apparatus for expanding cellular material by impregnating the cellular material with a liquefied gas at an elevated pressure, removing excess liquefied gas from the cellular material and causing the impregnated gas to vaporize, thereby expanding the cellular material. More particularly, the invention relates to a method of reducing the variation of the temperature of the liquefied gas in its storage vessel throughout the impregnation process.
2. Brief Description of the Prior Art
In the processing of cellular materials there is frequently a need or a desire to reduce the density of the material. For instance, when cellular products such as tobacco or other smoking products, fruits, vegetables, meats, cereals, and other food products are dried, they generally undergo shrinkage. It is often desirable to expand these products to modify their properties, such as burning characteristics, water absorbability, and appearance. Similarly, it is often desirable to expand other cellular materials such as cellulosic materials, for example sawdust, crushed fibre materials, etc. to increase their liquid absorbability or insulating properties.
A number of methods are known for expanding cellular materials. In general, these methods involve introducing an expanding agent, i.e. a substance capable of undergoing expansion, as by a phase change from a liquid to a gas, into the cells of the material, and causing the agent to expand. A particularly suitable method of expanding one of the above-described cellular materials comprises introducing a liquefied gas which is inert to the cellular material into a sealed vessel containing the material; soaking the cellular material in the liquefied gas sufficiently long to enable the liquefied gas to penetrate the cells of the material; removing excess liquefied gas from the cellular material; and expanding the cellular material by heating it, thereby vaporizing the liquefied gas within the cells of the material. The increased pressure of the vaporized liquefied gas in the cells causes the cellular material to expand.
It is also known to expand cellular material by impregnating it with a liquefied gas expanding agent, such as liquefied carbon dioxide, at an elevated pressure; removing excess expanding agent from the cellular material; reducing the pressure in the vessel in which the cellular material is contained, thereby causing the expanding agent to solidify; and heating the cellular material, such as by exposure to a hot gas stream, e.g. steam, air, etc., to cause the solidified expanding agent to evaporate or sublime. The solidified expanding agent vaporizes at a rate greater than the rate at which the agent in gaseous form can escape from the cellular material. As a result of this treatment, the cellular material is forced to expand.
U.S. Pat. No. 4,340,073 discloses a process and apparatus for expanding tobacco by impregnating the tobacco with carbon dioxide under conditions such that the carbon dioxide in contact with the tobacco is in liquid form, removing excess liquefied carbon dioxide from the tobacco, reducing the pressure of the impregnated tobacco to solidify carbon dioxide within the tobacco structure and rapidly heating the tobacco at atmospheric pressure to vaporize the carbon dioxide and expand the tobacco.
UK Patent Specification 1,484,536 discloses a particularly useful process for expanding an organic substance using liquid carbon dioxide. The process of this patent comprises the steps of pressurizing a vessel containing the substance to be expanded to a pressure in the range of about 200-1,070 psi with carbon dioxide, immersing the substance in liquid carbon dioxide while maintaining the pressure within the vessel, thereby impregnating the substance with the liquid carbon dioxide, removing excess liquid carbon dioxide from the impregnation vessel, depressurizing the vessel to substantially atmospheric pressure, thereby causing liquefied carbon dioxide on and in the substance to solidify, removing the impregnated substance from the vessel, and heating the substance to cause expansion of the substance by at least 10%. In the process of this patent specification, the carbon dioxide used to pressurize the impregnation vessel is taken from the vapor space of the process vessel that is used to provide liquid carbon dioxide to the impregnation chamber. It is further stated in this patent specification that after removal of the liquid carbon dioxide from the impregnation chamber, the impregnation chamber may be vented to the atmosphere or to a carbon dioxide recovery system.
U.S. Pat. No. 4,165,618 discloses a process and system similar to that disclosed in UK patent specification 1,484,536 using a liquid cryogen, such as liquefied carbon dioxide. In the process of this Patent, the vessel in which the tobacco is impregnated is purged and pressurized by transferring gas from the vapor space of a liquid cryogen storage vessel to the impregnating vessel. Subsequent to pressurization, liquid cryogen is transferred to the impregnation vessel from the liquid storage vessel. The tobacco is permitted to soak in the liquid cryogen for a predetermined time period, after which it is returned to the liquid storage vessel. The gaseous cryogen remaining in the impregnation vessel after removal of the liquid cryogen is then transferred to a series of accumulators from which the gas is compressed and eventually returned to the main reservoir for the liquid cryogen.
The processes disclosed in these patents have the disadvantage that the gas used to purge and pressurize the impregnation vessel comes from the vapor space of the liquefied gas storage vessel. Unfortunately, removal of gas from the vapor space of this vessel reduces the pressure in the vessel, thereby disrupting the equilibrium balance in the vessel and causing liquid in the vessel to boil. This in turn cools the liquid. In order to restore the pressure and temperature in the vessel to the desired equilibrium value, it is necessary to frequently heat the liquid in the storage vessel Heating the contents of the storage vessel to maintain temperature and pressure equilibrium of the liquid and vapor in the vessel significantly increases the cost of the impregnation process.
In processes such as the one disclosed in U.S. Pat. No. 4,165,618, discussed above, in which the vented gas is recovered and, after being compressed, is returned to the storage vessel, some of the heat lost from the liquid cryogen may be replaced by the return of the hot compressed gas to the storage vessel. The gas is hot because its temperature is raised upon compression. However, the additional heat returned to the storage vessel from the compressed gas is inadequate to maintain the desired pressure and temperature equilibrium in the storage vessel. Consequently, it is still necessary to heat the liquid in the storage vessel to re-establish temperature and pressure equilibrium.
U.S. Pat. No. 3,771,533 discloses impregnation of tobacco with a mixture of ammonia and carbon dioxide, which react to form ammonium carbonate. The ammonium carbonate is thereafter decomposed by heat to release gases which expand the impregnated tobacco. U.S. Pat. No. 4,165,618 discloses the use of a variety of liquefied inert cryogens, including nitrogen, oxygen, argon, hydrogen, helium, methane, the freons, carbon monoxide and carbon dioxide as impregnants in the expansion of tobacco and like products.
In order to improve the economics of liquefied gas impregnation process, improvements that will lower the cost of processing, are continuously sought. The present invention provides an improvement that reduces the costs of impregnation processes.